Project Summary Growth factor independence 1 (Gfi1) is a zinc-finger transcriptional repressor that plays a critical role in hematopoiesis. When aberrantly activated, Gfi1 functions as a dominant oncoprotein that collaborates with c- Myc and other oncoproteins in lymphomagenesis. TGF? is a secreted cytokine that inhibits cell proliferation and acts as a tumor suppressor in the hematopoietic system. TGF? induces the expression of CDK inhibitors p21Cip1, p15INK4B, p27Kip1 and p57Kip2, and this effect is mediated at least in part through rapid downregulation of c-Myc, which releases Miz-1 from the Myc/Miz-1 complex to activate Miz-1 target genes, including the different CDK inhibitors. We previously showed that Gfi1 forms a Gfi1/Miz-1/c-Myc ternary complex through interacting with Miz-1 and act in collaboration with c-Myc to repress p15INK4B, p21Cip1 and p27Kip1. In preliminary studies, we demonstrate that Gfi1 inhibits the cytostatic effect of TGF? and stabilizes c-Myc protein. Notably, Gfi1 also upregulates c-Myc mRNA level. As Gfi1 is a transcriptional repressor, it is less likely that Gfi1 directly activates c-Myc transcription. Our preliminary data identify Kruppel-like factor 6 (Klf6), a tumor suppressor and important regulator of hematopoiesis, as a likely novel Gfi1 target gene. Interestingly, Klf6 has been shown to suppress the expression of c-Myc. Conversely, TGF? inhibits the expression of Gfi1 and abolishes Gfi1-mediated upregulation of c-Myc in hematopoietic cells. In this proposal, we will examine how Gfi1 functionally interacts with TGF? and c-Myc to regulate cell proliferation and oncogenic transformation in hematopoietic cells. In aim 1, we will further evaluate the role of Gfi1 in the regulation of cellular response to TGF?. We will examine whether Gfi1 overexpression or deficiency has an effect on the cytostatic effect of TGF? in mouse Lin- bone marrow (BM) cells and whether interaction with Miz-1 is required for Gfi1 to counteract the cytostatic activity of TGF?. We will further examine whether homozygous and heterozygous deletions of Gfi1 affect leukemogenesis arising from loss of TGF? signaling. In aim 2, we will investigate whether Gfi1 stabilizes c-Myc protein through interacting with Miz-1 and/or competing with c-Myc for a common E3 ubiquitin ligase. We will also address how TGF? abolishes c-Myc upregulation by Gfi1, and further assess the role of Klf6 in Gfi1- mediated upregulation of c-Myc mRNA level. In aim 3, we will determine the mechanism of Gfi1 downregulation by TGF?. Specifically, we will address whether TGF?-activated Smad proteins directly bind to Gfi1 to repress its expression. The proposed research will provide critical insights into the molecular mechanism by which aberrant activation of Gfi1 leads to hematopoietic malignancies and the mechanism whereby TGF? acts as a tumor suppressor in the hematopoietic system.